FIG. 1 illustrates a simplified block diagram of an equipment configuration for one terminal of a communication link which includes a near end hybrid. The communication link has a near-end 8 comprising a telephone 2, a four-to-two wire hybrid circuit 3, and an echo canceller circuit 4. A far-end connected to communication network 23, can be similarly configured but is not illustrated in FIG. 1. During a conversation between the near-end and far-end users, the far end signal, X, which contains both the far-end user's speech and incidental background noise, enters the near-end 8 as signal X at node 9.
The far-end signal is provided to the four-to-two wire hybrid circuit 3 and then to near-end telephone 2. Due to the unavoidable non-linearities present in the hybrid circuit 3, some portion of the far-end signal power is coupled onto the output 7 of the hybrid circuit 3 as an echo. A composite signal Y exists at node 7 containing the echo signal and the combined speech of the near-end user and any incidental background noise from the near-end user's environment. A filter having a filter length period selected and designed to be longer than the hybrid dispersion time is used prior to power level measurements at 7 to allow the echo canceller 4 to operate properly.
Echo canceller 4 synthesizes the expected value of the echo signal and subtracts this value from the composite signal Y existing at node 7. The resulting difference signal, e, existing at node 14, is intended to contain only the near-end signal originating from telephone 2. Ultimately, difference signal, e, is provided to the far-end telephone through the communications network 23.
Methods of measuring the echo return loss typically measure a signal at node 9, where the signal power from the far-end would normally exist. A measurement of the signal power, X, at node 9 is made. Additionally, the power level of the composite signal Y, comprised of the coupled echo signal and any signal generated by the near-end telephone 2, is measured at node 7. The measurement can be made when little-to-no signal is being generated at near end telephone 2. Assuming the signal power of any signal generated by the near-end telephone is very small in comparison to the coupled echo signal power, the ratio of the measured test signal power X to the measured power level Y provides an estimate of the echo return loss (ERL) for the near-end 8. The magnitude of echo return loss is usually measured as a difference in dB between signal X and signal Y. As described in the co-pending application Ser. No. 10/029,669, incorporated herein in its entirety by reference, echo return loss may be measured dynamically during the course of a telephone conversation.
Echo is an important factor in communications which include a hybrid between a four wire communication network 23 and the end terminals 24 and 25 as illustrated in FIG. 2. When echo is present, it is preferable to eliminate the echo. To eliminate the echo, the magnitude of the echo must be determined. One way of determining the magnitude of the echo is through echo return loss (ERL) estimation. A high echo return loss means that there is very little echo because most of the energy from the far end has been lost when the near end signal combined with echo is measured.
If one or more of the terminal units 28 is a four wire unit and is connected by four wires directly to the four wire communication network 23, as illustrated in FIG. 3, then the other unit 25 will not experience hybrid induced echo. If both of the terminal units are four wire units connected as four wire units, then neither unit will experience hybrid induced echo. If no echo is present, no echo cancellation is needed.